1. Field of the Invention
The present invention relates to an optical disc comprising on a substrate a recording layer including at least a phase change film changing between a crystalline state and amorphous state, so that this recording layer is heated by a ray radiation so as to cause a phase change for recording and deleting an information. More specifically, the present invention relates to an improvement of materials constituting a light reflection film formed on the recording layer.
2. Description of the Prior Art
As an optical disc for recording and/or reproducing information signals by applying a laser beam, there can be exemplified an optical disc in which an information signal is written beforehand by an emboss pit; a phase change optical disc for writing an information signal by utilizing a phase change between a crystalline state and an amorphous state of a recording layer; and a magneto-optical disc for writing an information signal by utilizing the magneto-optical effect of a recording layer. These optical discs have function films such as a recording layer and a light reflection film on a transparent substrate made from a plastic such as polycarbonate or glass.
In the case of the phase change optical disc, recording, erase, and reproduction are performed as follows. Firstly, this phase change optical disc is subjected to an initialization treatment by applying a predetermined laser beam so that the recording layer is uniformly made into a crystalline state. When recording an information signal on this optical disc, a laser beam of a high-level power (hereinafter, referred to as a recording power) is applied to the recording layer so as to increase the temperature of the recording layer equal to or above its melting point, after which the recording layer is suddenly cooled down so that the portion of the recording layer to which the laser beam has been applied becomes a record mark of the amorphous state.
Moreover, when erasing an information signal which has been recorded on the recording layer, a laser beam of a weaker power than the record power is applied to at least the record mark so that the portion to which the laser beam is applied is heated equal to or above the crystallization temperature but below the melting temperature, after which the portion is suddenly cooled down so that the record mark of the amorphous state becomes crystalized.
Furthermore, in this phase change optical disc, the reflection factor differs between the crystalline state and the amorphous state of the material constituting the recording layer. Accordingly, when reproducing the information signal, the weakest laser beam is applied to the recording layer so as to detect the reflection factor change in the respective states.
Thus, the phase change optical disc does not require means for generating an external magnetic field and accordingly, the size of a recording/reproduction apparatus can be reduced. Moreover, the phase change optical disc can be re-written and produced. This phase change optical disc attracts much attention now.
The recording layer of this phase change optical disc may be made from a phase change material such as a chalcogen compound containing Ge such as Ge--Sb--Te (hereinafter, referred to as Ge chalcogen compound) and a chalcogen compound containing Ag such as Ag--In--Sb--Te (hereinafter, referred to as Ag chalcogen compound). Especially, a phase change optical disc having a recording layer made from the Ge chalcogen compound is known to exhibit a preferable repeated recording characteristic and an excellent repeat durability.
However, even in this phase change optical disc, a further quality improvement is desired as a recording medium of the next generation and there arises following problems.
The phase change optical disc using the Ge chalcogen compound as the material of the recording layer shows a tendency to have a high jitter value of a reproduction signal. Although the phase change disc using the Ge chalcogen compound as, the material of the recording layer has a durability superior to the phase change optical disc using the Ag chalcogen compound, the jitter value suddenly increases as the recording layer is deteriorated after repeated recording of several tens of thousand times. For this, in this phase change optical disc, after repetition of recording a number of times, only a very narrow recording power margin can be obtained for suppressing the jitter value of the reproduction signal below the reproduction limit level.
That is, in this phase change optical disc, after repetition of recording a number of times, the recording power margin enabling to obtain a preferable reproduction signal becomes very narrow.
As a result, in this phase change optical disc, after repetition of recording a number of times, the recording/reproduction failure is easily caused by the recording power fluctuation and the ambient temperature change.